1. Field of the Invention
The present invention relates to a method of imaging steep-sloping geologic interfaces such as faults or salt dome flanks, using seismic reflections referred to as prismatic reflections.
2. Description of the Prior Art
Prismatic reflections are understood to be double reflections whose impact points are located on the two faces, not necessarily in a plane, of a dihedron, thus forming a ray with three “branches”. Each dihedron is a steep-sloping interface resulting from a structural irregularity of the subsurface (a fault for example) and “primary reflectors”. “Primary reflectors” are understood to be the interfaces corresponding to the boundaries of sedimentary deposits. Prismatic reflections therefore carry information on the geometry of the steep-sloping interface. Like any reflection observed for various source-pickup pairs, prismatic reflections also carry information on the velocity distribution in the subsurface.
Reflection shooting is widely used for oil exploration. As the name indicates, to provide images of the subsurface, reflection shooting extracts the information contained in the seismic events that undergo a single reflection on the geologic interfaces. Primary reflection events are of the type where the reflection from the interface is a primary reflector. Other events, in particular multiple reflections, are often considered to be noise likely to disturb seismic imaging techniques. It is therefore generally desired to eliminate multiple reflections.
Primary reflections generally do not allow obtaining an image from steep-sloping events such as faults and salt dome flanks, especially if they are overhanging. For example, in FIG. 1 showing a section extracted from a poststack 3D migrated volume, a salt dome can be recognized. The position of the top of the dome can be clearly seen. On the other hand, the position of the flanks and of the base is indefinite. This FIG. 1 illustrates the limits encountered by conventional processing based on the use of primary reflections for imaging the flanks of a salt dome. Certain uses of diving waves, which have sometimes been used to image such structures in the Gulf of Mexico as described, for example, in U.S. Pat. No. 5,235,555, can however be noted. However, the existence of such waves requires a particular configuration of the velocity distribution in the subsurface.
Prismatic waves, which are a particular case of multiple reflections, are encountered much more commonly, notably in geometries where the geologic interfaces are steep-sloping. Various examples of such events are for example described in the following publication:
Hawkins, K., 1994, “The Challenge presented by North Sea Central Graben Salt Domes to all DMO Algorithms”: First Break, 12, No. 6, 327-343. They are considered to be noise to be eliminated.
Using the information contained in these prismatic reflections in order to obtain information on the geometry of steep-sloping interfaces has already been considered by D. W. Bell in U.S. Pat. No. 4,987,561 in cases where the primary interfaces and the sloping reflectors are in a plane and are perpendicular to one another.
Another approach, which is complex to implement because of its lack of reliability, has also been considered by P. Lailly and K. Broto (“Towards the Tomographic Inversion of Prismatic Reflections”, 711st Ann. Internat. Mtg., Soc. Expl. Geophys., Expanded Abstract, 726-729, 2001). It is based on the well-known reflection traveltime tomography technique, described for example in the following document:
Bishop, T., et al., 1985, “Tomographic Determination of Velocity and Depth in Laterally Varying Media”, Geophysics, 50 No. 6, 903-923.